Thanks for the link. Currently, I am working in an old installation, Word 2019 running under Windows, but the principles remain the same.
Double-clicking the header area activates the header/footer view. If you display nonprinting marks (click to activate the ¶ icon on the Home tab) you will see that the watermark is anchored to the page header. You can click to select the object.
In Word 2019, the selected watermark has a dotted selection border (or actually with tiny squares); you may see something slightly different in your version of Word for Mac. After it has been selected, you can Cut the object to remove it.
Note that since the document has multiple unlinked headers, you have to repeat the procedure, once per header type (three time should do the trick).